Earth was always dynamic! Earth has changed many times over billions of years. Huge mountains have formed, been destroyed, and been replaced with new mountains. Continents have moved, split apart and collided with each other. Ocean basins have opened up. Life on Earth evolved slowly for billions of years.

Early Continents

The earliest crust was probably basalt. It may have resembled the current seafloor. This crust formed before there were any oceans. More than 4 billion years ago, continental crust appeared. The first continents were very small compared with those today.

Continents Grow

Continents grow when
microcontinents
, or small continents, collide with each other or with a larger continent. Oceanic island arcs also collide with continents to make them grow.

Cratons

The earliest continental crust is now found in the ancient cores of continents, called the
cratons
. Geologists can learn many things about the Precambrian by studying the rocks of the cratons.

Cratons contain felsic igneous rocks, which are remnants of the first continents.

Cratonic rocks contain rounded sedimentary grains. Rounded grains indicate that the minerals eroded from an earlier rock. It also means that rivers or seas existed.

One common rock type in the cratons is
greenstone
, a metamorphosed volcanic rock (
Figure
below
). Since greenstones are found today in oceanic trenches, what does the presence of greenstones mean? These ancient greenstones indicate the presence of subduction zones.

Ice age glaciers scraped the Canadian Shield down to the 4.28 billion year old greenstone in Northwestern Quebec.

Supercontinents

There are times in Earth history when all of the continents came together to form a
supercontinent
. Supercontinents come together and then break apart. Pangaea was the last supercontinent on Earth, but it was not the first. The supercontinent before Pangaea is called Rodinia. Rodinia contained about 75% of the continental landmass that is present today. The supercontinent came together about 1.1 billion years ago. Rodinia was not the first supercontinent either. Scientists think that three supercontinents came before Rodina, making five so far in Earth history.

Early Plate Tectonics

Since the early Earth was very hot, mantle convection was very rapid. Plate tectonics likely moved very quickly. The early Earth was a very active place with abundant volcanic eruptions and earthquakes. The remnants of these early rocks are now seen in the ancient cores of the continents.

Vocabulary

greenstone
: Metamorphosed ancient oceanic crust.

microcontinent
: Small continents that come together to become part of larger continents.

supercontinent
: A very large continent created by several continents and microcontinents.